Belize Yacht Club has a conference facility that can handle your needs. They have two rooms set up for that called the Coral Sands Convention Center. Very nice and has catering ect. available. It's on the beach just south of town. I think they have a link on this site or you can ask Marty how to get in touch.
Journey's End has one as well. We were there last week and a group of divers had slide presentation. There were at least 40 people, not fancy but good size. A/c with sand floor, very island like.
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